The present invention relates to a method for etching an organic/inorganic hybrid film represented by SiCxHyOz (x>0, y≧0, z>0), a semiconductor device having an interlayer insulating film made of the organic/inorganic hybrid film, and a fabricating method for such a semiconductor device.
Recent semiconductor integrated circuit devices adopt multilayer interconnection structures to meet requests for size scale-down and higher integration. Conventionally, a silicon oxide (SiO2) film has been used as an interlayer insulating film provided between lower interconnections and upper interconnections. Contact holes are formed through such an interlayer insulating film by plasma etching for connection with lower interconnections when a multilayer interconnection structure is adopted.
Hereinafter, as a first conventional example, an etching method for formation of contact holes through an interlayer insulating film made of a silicon oxide film will be described with reference to FIGS. 22(a) to 22(d).
First, as shown in FIG. 22(a), a lower interconnection 12 made of copper, for example, is formed in an insulating film 11 deposited on a semiconductor substrate 10 by a known method. On the lower interconnection 12, deposited is an etching stopper film 13 made of a silicon nitride (Si3N4) film, for example, that has the function of preventing the lower interconnection 12 from oxidizing during etching and also stopping the etching. An interlayer insulating film 14 made of a silicon oxide (SiO2) film is deposited on the etching stopper film 13. A resist pattern 15 having an opening for formation of a contact hole is then formed on the interlayer insulating film 14. Note that, although illustration is omitted, the sides and the bottom of the lower interconnection 12 are normally coated with barrier metal.
Thereafter, as shown in FIG. 22(b), a contact hole 16 is formed through the interlayer insulating film 14 using the resist pattern 15 as a mask by plasma etching with an etching gas containing fluorine and carbon, such as CF4 gas, C2F6 gas, C3F8 gas, CHF3 gas, C3F8 gas, or C4F8 gas.
As shown in FIG. 22(c), the resist pattern 15 is removed by ashing with oxygen plasma. As shown in FIG. 22(d), the portion of the etching stopper layer 13 exposed in the contact hole 16 is removed.
In recent years, further scale-down and higher integration of multilayer interconnection structures have been demanded, and with realization of this demand, signal delay at interconnections has become greatly influential to the operation speed of a semiconductor integrated circuit.
In order to reduce signal delay at interconnections, it has been proposed to use a film having a low dielectric constant (ε=2 to 3) as the interlayer insulating film. As such a film having a low dielectric constant, known are an organic insulating film containing an organic compound as a main component, a fluorine-containing insulating film made of a fluorine-containing silicon oxide (SiOF), and an organic/inorganic hybrid film represented by SiCxHyOz (x>0, y≧0, z>0). Japanese Laid-Open Patent Publication No. 10-125674 proposes an organic/inorganic hybrid film made of a silicon oxide film containing carbon and hydrogen, deposited by feeding hexamethyldisiloxane (HMDSO) as a material gas.
The organic insulating film, of which the composition is similar to that of a resist film, has the following problem. When a resist pattern formed on the organic insulating film is to be removed by ashing with oxygen plasma, the organic insulating film itself is damaged by the oxygen plasma. The fluorine-containing insulating film has the problem that it easily comes off due to its poor adhesion to an underlying film and also it is poor in mechanical strength and heat resistance.
The organic/inorganic hybrid film has a specific dielectric constant considerably smaller than the fluorine-containing insulating film and has a mechanical strength roughly equal to that of the fluorine-containing insulating film. Moreover, the organic/inorganic hybrid film, of which the composition is not similar to that of a resist film, is less damaged by oxygen plasma, and therefore, the resist pattern can be removed by ashing with oxygen plasma.
In consideration of the above, the organic/inorganic hybrid film is promising as an interlayer insulating film having a low specific dielectric constant.
With the recent demand for size scale-down and higher integration of semiconductor integrated circuit devices, also, the diameter of contact holes formed through the interlayer insulating film has become finer and the aspect ratio of the contact holes has become larger. It is difficult to fill such fine contact holes having a large aspect ratio with a conductive material with reliability.
To solve the above problem, Japanese Laid-Open Patent Publication No. 8-191062, for example, proposes a technique in which the diameter of the contact holes is made larger near the opening thereof than near the bottom thereof, to facilitate filling of the contact holes with a conductive material.
Hereinafter, as the second conventional example, the etching method disclosed in Japanese Laid-Open Patent Publication No. 8-191062 will be described with reference to FIGS. 23(a) to 23(d). Note that in FIGS. 23(a) to 23(d), illustration of a lower interconnection is omitted.
First, as shown in FIG. 23(a), a resist pattern 15 having an opening 15a for formation of a contact hole is formed on an interlayer insulating film 14 made of a silicon oxide film deposited on a semiconductor substrate 10.
As shown in FIG. 23(b), the interlayer insulating film 14 is subjected to anisotropic dry etching with an etching gas containing fluorine and carbon using the resist pattern 15 as a mask, to form a contact hole 16 to reach partway in the interlayer insulating film 14.
Isotropic dry etching is then performed for the interlayer insulating film 14 with an etching gas including oxygen gas. By this etching, as shown in FIG. 23(c), an opening 15a of the resist pattern 15 is widened, and with this, the diameter of the contact hole 16 is made larger near the opening thereof, to provide a tapered wall at the opening of the contact hole 16.
As shown in FIG. 23(d), the resist pattern 15 is removed. Although illustration is omitted, by depositing a conductive material on the interlayer insulating film 14, the contact hole 16 is filled with the conductive material with reliability.
(First Problem)
The plasma etching for forming fine contact holes through an organic/inorganic hybrid film is normally performed with an etching gas containing fluorine and carbon, which can cleave Si—O bonds, as in the plasma etching of a silicon oxide film.
However, when the organic/inorganic hybrid film is etched with the same etching gas under the same conditions as those used for etching of the silicon oxide film, the etching rate largely decreases, or in an extreme case, the etching itself stops. The decrease in etching rate causes reduction in throughput. This also causes reduction in the difference between the etching rate of the interlayer insulating film and that of the resist pattern, failing to secure a sufficiently large etching selection ratio.
By adding oxygen gas to the etching gas, the etching rate of the organic/inorganic hybrid film increases. However, this also facilitates etching of the resist pattern 15, and thus the etching selection ratio of the interlayer insulating film 14 to the resist pattern 15 decreases.
The addition of oxygen gas to the etching gas also increases the etching rate of the silicon nitride film constituting the etching stopper film 13. This reduces the etching selection ratio of the interlayer insulating film 14 to the etching stopper film 13.
Therefore, it is not preferable to add oxygen gas to the etching gas.
In view of the above, the first object of the present invention is providing good plasma etching for an organic/inorganic hybrid film.
(Second Problem)
As described above, the etching stopper film 13 made of silicon nitride film is deposited on the lower interconnection 12 made of a copper film, for example. The specific dielectric constant of the silicon nitride film is about 7, which is significantly large compared with the specific dielectric constant of the organic/inorganic hybrid film.
Having such an etching stopper film, therefore, the reduction in specific dielectric constant between the upper and lower interconnections is not sufficiently attained despite of the formation of the interlayer insulating film 14 made of the organic/inorganic hybrid film in an attempt to reduce the specific dielectric constant.
In view of the above, the second object of the present invention is reducing the specific dielectric constant between the upper and lower interconnections by reducing the specific dielectric constant of the etching stopper film.
(Third Problem)
The second conventional example described above is an etching technique in which the resist film is etched more isotropically to widen the openings of the resist film by adding oxygen gas to the etching gas, to thereby provide contact holes having a tapered opening. However, this technique requires a large amount of etching of the resist film, and therefore it is not possible to increase the thickness of the resist film in an attempt to form contact holes having a large aspect ratio. For this reason, the second conventional example finds difficulty in application to formation of contact holes having a large aspect ratio. In particular, in the case of forming tapered contact holes through the interlayer insulating film made of an organic/inorganic hybrid film, how the etching amount of the resist film should be reduced is a big problem to be solved.
There is also reported a technique in which the contact holes are etched into a tapered shape using an etching gas containing fluorine and carbon without changing the diameter of the openings of the resist film. However, whether or not this technique is applicable to the formation of contact holes through the interlayer insulating film made of an organic/inorganic hybrid film has not been verified.
In view of the above, the third object of the present invention is providing a method in which contact holes having an increased diameter near the opening thereof can be formed through an interlayer insulating film made of an organic/inorganic hybrid film with reliability.
(Fourth Problem)
In recent years, in order to enhance the resolution between light exposed portions and non-exposed portions of a resist film, there has been proposed a technique of forming a resist pattern using a chemical amplification resist material. According to this technique, the polarity (solubility to a developer) is changed in portions of the resist film made of a chemical amplification resist material exposed to an energy beam by the function of acid generated in the exposed portions. The exposed portions or non-exposed portions are then removed with the developer, to form a resist pattern.
The present inventors formed a resist film by applying a chemical amplification resist material to an organic/inorganic hybrid film, and subjected the resist film to pattern light exposure. As a result, it was found that exposed portions of the resist film failed to sufficiently change the polarity presumably due to a reduced amount of acid generated in the exposed portions. Therefore, the resultant resist pattern after removal of the exposed portions or non-exposed portions of the resist film with a developer was faulty in shape.
The present inventors attempted to increase the exposure amount during the pattern light exposure, but failed to sufficiently change the polarity of the exposed portions of the resist film.
The faulty formation of the resist pattern did not occur when a chemical amplification resist film was formed on a silicon oxide film, but was unique to the chemical amplification resist film formed on an organic/inorganic hybrid film. The faulty formation of the resist pattern was confirmed to occur when using a positive chemical amplification resist film, but is presumed to also occur when using a negative chemical amplification resist film.
Hereinafter, a problem occurring in the formation of multilayer interconnections having a dual damascene structure, which uses a chemical amplification resist pattern formed on an organic/inorganic hybrid film, will be described with reference to FIGS. 24(a), 24(b), and 25.
First, as shown in FIG. 24(a), a lower interconnection 22 is formed on an insulating film 21 deposited on a semiconductor substrate 20. An etching stopper film 23 is deposited on the lower interconnection 22, and then an interlayer insulating film 24 made of an organic/inorganic hybrid film is deposited on the etching stopper film 23. Thereafter, a contact hole 25 is formed through the interlayer insulating film 24 by plasma etching using a first resist pattern that is formed on the interlayer insulating film 24 and has an opening for formation of the contact hole.
A chemical amplification resist material is then applied to the resultant interlayer insulating film 24 to form a resist film. The resist film is then subjected to pattern light exposure and development, to form a second resist pattern 26 having an opening for formation of an interconnection groove. At this stage, the resist film partly remains after the above processing, forming a resist film 26a over the top surface of the interlayer insulating film 24 as well as the wall and the bottom of the contact hole 25. The reason why the resist film 26a is formed is considered that acid has been reacted with some reactive group and consumed.
Thereafter, the interlayer insulating film 24 is subjected to plasma etching using the second resist pattern 26 as a mask, to form an interconnection groove 27 in the interlayer insulating film 24 as shown in FIG. 24(b). During this etching, a barrier (inner crown) 28 made of the interlayer insulating film 24 is formed since the resist film 26a on the inner side of the interconnection groove 27 serves as a mask.
After removal of the second resist pattern 26 and the resist film 26a as shown in FIG. 25, the contact hole 0.25 and the interconnection groove 27 are filled with a conductive material to form a plug and an upper interconnection. At this time, due to the existence of the barrier 28 on the inner side of the interconnection groove 27, the contact resistance between the upper interconnection embedded in the interconnection groove 27 and the plug embedded in the contact hole 25 disadvantageously increases.
In view of the above, the fourth object of the present invention is preventing deactivation of acid in a chemical amplification resist film formed on an organic/inorganic hybrid film, to improve the resolution of the resist film.